16 
BULLETIN 1166, U. S. DEPARTMENT OF AGRICULTURE. 
value, or H + concentration, is stated in terms based on the H + con- 
centration of pure neutral water 23 as unity. It may be stated directly 
in whole numbers as "specific acidity," or, as is done more frequently, 
as the logarithm of the reciprocal of the actual concentration, gen- 
erally designated by the symbol pH. 
The data on the specific acidity of extracts of the materials used 
in this investigation (Table 8) are of value not only in determining 
the " strength " of the acid-reacting compounds in the apple products 
but also in indicating the identity of these " acids." 
Table 8. 
-Acidity of extracts of dried apple pomace, dried pectin pulp, com 
silage, and dried beet pulp. 
Product extracted. 
Dried apple pomace (80 per cent alcohol ex- 
tract) 
Dried apple pomace (400-pound sample): 
80 per cent alcohol extract 
Aqueous extract 
Corn silage (moisture content, 71.4 per cent) 
(80 per cent alcohol extract) 
Corn silage (moisture content, 75.3 per cent) 
(80 per cent alcohol extract) 
Dried apple-pectin pulp (400-pound sample): 
80 per cent alcohol extract 
Aqueous extract 
Dried apple-pectin pulp (unground) (80 per 
cent alcohol extract) 
Dried apple-pectin pulp (2-ton sample) (80 
per cent alcohol extract) 
Dried beet pulp: 
80 per cent alcohol extract 
Aqueous extract 
Sam- 
ple 
No. 
37199 
37254 
36873 
37157 
36675 
37373 
37866 
37867 
Quan- 
tity of 
sample 
per 100 
cubic 
centi- 
meters 
of ex- 
tract. 
Grams. 
13.3 
l.o 
5. 
9.3 
10.0 
10.0 
4.0 
4. 
4.0 
5.0 
Total 
acidity 
per 100 
cubic 
centi- 
meters 
of ex- 
tract. 
Cc N 
acid. 
2.505 
.928 
.667 
3.100 
3.240 
2.420 
1.900 
1.248 
.992 
.192 
.105 
Hydrogen-ion concentration. 
Of extract 
examined. 
(Observed.) 
Specific 
acidity. 2 
10,000 
10,000 
10,000 
2,500 
5,000 
6,300 
4,000 
10,000 
6,300 
6,300 
pH. 
3.0 
3.0 
3.0 
3.6 
3.3 
3.2 
3.4 
3.0 
3.2 
3.2 
4.1 
Of aqueous 
solution of 
"pure" acid 
of same total 
acidity. 1 
Acid 
potas- 
sium 
malate 
(HKC 4 - 
H 4 5 ). 
(Com- 
puted.) 
pH. 
3.36 
3.58 
3.65 
3.37 
3.42 
3.52 
3.57 
Malic 
acid 
(H 2 C 4 - 
H4O5). 
(Com- 
puted.) 
2.5 
2.63 
2.72 
2.39 
2.45 
2.56 
2.61 
* Computed for an aqueous solution of "pure" acid having the same total acidity as the extract examined. 
2 In round numbers. 
Bigelow and Dunbar (44) have shown that free malic acid is the 
source of acidity in apples. The treatment to which the fruit is sub- 
jected in obtaining dried pomace, however, permits of fundamental 
changes in the character of the " acidity " during the process of 
manufacture. 
By comparing the pH exponents (Table 8) determined colorimetri- 
cally on the dried pomace extracts with those computed for pure 
solutions of acid potassium malate and free malic acid, it is ap- 
parent that if malic acid is principally responsible for the acidity, a 
23 " Neutral " water as measured by its conductivity. Careful experimentation! has 
shown that a liter of such water dissociates to the extent of containing 0.0O0O001 gram 
of H + or 10- 7 gram; hence its pH value (the pH of the neutral point) is 7. 
